Ship stabilization systems



Nov. 28, 1967 F. J. NICKELS, JR 3,354,859

SHIP STABILIZATION SYSTEMS Filed July 1, 1966 3 Sheets-Sheet 2 INVENTOR.

FRANK J. NICKELS, JR.

ATTORNE Nov. 28, 1967 F. J. NICKELS, JR

SHIP STABILIZATION SYSTEMS 5 Sheets-Sheet 5 Filed July 1, 1966 FIG. 7

FIG. 8

INVENTOR. FRANK J. NICKELS, JR. 9M4 W I ATTORNE United States Patent r 3,354,859 SHIP STABILIZATION SYSTEMS Frank J. Nickels, Jr., Swarthmore, Pa., assignor to Sun Shipbuilding & Dry Dock Company, Chester, Pa., a corporation of Pennsylvania Filed July 1, 1966, Ser. No. 562,274 2 Claims. or. 114-425 ABSTRACT OF THE DISCLOSURE A passive anti-rolling tank system for stabilizing ships comprises two liquid-containing tanks (or one tank separated into two portions by means of a central partition) on opposite sides of the longitudinal center line of the ship, connected together by a liquid crossover passage and also by an air crossover passage. A removable orifice plate is inserted into the air crossover passage, and the orifices may be changed to vary the rate of liquid transfer between the tanks. A plurality of air crossover passages may be used between the tanks, each having a removable orifice plate therein.

This invention relates to stabilization systems for marine vessels (ships), and more particularly to ship stabilization systems wherein passive anti-rolling tanks are utilized.

In oceangoing vessels, the need for combating ship motion due to wave action has long been realized. There have been a variety of devices and designs directed to reducing undesirable motion in a seaway. Roll stabilization, the aim of the present invention, has continued to be a problem to be reckoned with, although various devices for reducing roll are in operation today on many types of vessels.

Systems of passive tanks for shifting the center of gravity of a vessel, to create a righting moment to rolling or heeling action, have been tried. Such stabilization systems utilized, generally, a pair of closed wing tanks, one located on each side of the ship, these tanks being connected at the bottoms by a crossover tube or duct to form a U-shaped arrangement which was completely filled with water while in operation, and also by an air duct connected between the tops of the tanks and having therein a valve for controlling the flow of air between the tanks.

These systems were tried on a number of vessels, but

- were found to suffer from several disadvantages. One disadvantage was that the valve in the air duct is relatively expensive. Another was that the valve is subject to wear, and still another was that the amount of opening of the valve is uncertain.

The systems just described require that the lowest level of water in a tank during transfer could not fall below the top of the water crossover duct; otherwise, the damping of the system would not be operable. At the same time, space considerations usually dictate that the stabilizer be kept to one deck height, whenever the installation is to be below the main deck. These requirements severely limit what the designer may do to get best use of the weight and space that reasonably can be allowed for passive anti-rolling tanks. When the designer chooses a conventional U-tube type stabilizer he is confronted with the difficulty that, of the Water in the tank, the portion between the bottom of the tank and the level corresponding to the top of the crossover duct is water which serves no really useful purpose. This Water that adds nothing to the effectiveness of the stabilizer does, hoW- ever, add to the required weight and space of the instal lation.

3,354,859 Patented Nov. 28, 1967 In other passive-type stabilization systems which have been tried, the separate air duct or air connection was eliminated; this was accomplished by extending the water crossover duct to substantially the level of the tank tops and making the crossover or transfer duct into a flume. However, such systems cannot readily be tuned, that is, in such stabilization systems the frequency of Water transfer cannot readily be varied. This is a drawback.

In operation, the anti-rolling tank is partially filled with water. As the seaway causes the ship to roll, the water will tend to run from side to side of the tank in its own natural period. Stabilization is obtained when the slope of the water surface in the tank lags the roll angle of the vessel by approximately one-quarter cycle, thus providing a weight of water (and corresponding stabilizing moment) in a direction opposite to that in which the ship is tending to roll. Since the roll amplitude response of ships is most pronounced over a narrow band at the ships natural rolling frequency, tuning the tank system to this same frequency will effectively eliminate the greater portion of the objectionable high amplitude rolling. A vessels natural rolling frequency depends on its mass radius of gyration and its stability measure, both of which are variable with cargo loading, so it may be seen that it is highly desirable to be able to adjust the tanks natural frequency to that of the vessel at the time of each change of loading. This feature, tuning the stabilization system to suit each loading condition, is not available with the stabilization systems described in the preceding paragraph.

An object of this invention is to provide novel stabilization systems for marine vessels.

Another object is to provide ship stabilization systems which are economimal in cost, weight, and space.

A further object is to provide a passive anti-rolling tank system for ships which can be readily tuned to suit each loading condition of the ship.

A still further object is to provide a passive anti-rolling tank system wherein the total weight of water necessary for stabilization is minimized.

The objects of this invention are accomplished, briefly, in the following manner:

An elongate enclosure extends transversely across a marine vessel to be stabilized, and in this enclosure is a body of liquid which partially fills the same. A centrally-located imperforate plate separates the air space above the liquid into two portions, without interfering with the liquid flow in said enclosure. An air duct spans the plate and is connected between the two air space portions, and in this latter duct there is a removable orifice plate which provides a throttling effect on the air passing therethrough. The liquid crossover passage, in the (central) regionof the plate, is dropped below the level of the remainder of the enclosure. Several air ducts may be used, each having a removable orifice plate therein, and in modification separate wing tanks may be used at the sides of the ship, connected at their bottoms by a liquid crossover duct and at their tops by one or more air ducts with removable orifice plates therein.

A detailed description of the invention follows, taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a diagrammatic cross-sectional representation of a ship provided with a stabilization system according to this invention;

FIG. 2 is a view, on an enlarged scale and in detail, of a portion of FIG. 1;

FIG. 3 is a sectional view taken on line 3-3 of FIG. 2;

FIG. 4 is a face view of an orifice plate assembly;

FIG. 5 is a sectional view taken on line 5-5 of FIG. 1;

FIG. 6 is a sectional view taken in the direction indicated by line 66 of FIG. 1, but illustrating a modification;

FIG. 7 is a diagrammatic view similar to FIG. 1, but illustrating a modification; and

FIG. 8 is a top plan view of a modified ship stabilization system.

Referring first to FIGS. 1-5, a typical ship 1 is illustrated afloat in a seaway 2. A passive anti-rolling tank 3 extends entirely across ship 1, preferably at the top of the hull, as illustrated in FIG. 1. This tank has the same length dimension (measured parallel to the length dimension of the ship) throughout its extent (see FIG. The tank 3 is of rectangular cross-section, as shown in FIG. 5.

At the central point of the width of tank 3 (and thus also on the longitudinal center line of the ship) an imperforate planar baffie plate 4 is mounted within this tank. Baflie 4 extends downwardly from the top wall of tank 3 but has a height less than that of the tank, thus leaving a water crossover passage 5 between the bottom edge of the battle and the bottom of the tank. Baflle 4 has a length dimension (again measured parallel to the length dimension of the ship) equal to that of the tank 3, and is sealed at its edges to the top wall and side walls of the tank, in an air-tight manner.

The baflie 4 is contained in the plane which also contains the longitudinal axis of the ship 1. In effect, the baffle 4 divides the tank 3 into two tank portions which are coupled together by the water crossover passage 5. The height dimension of the tank 3 is increased at the center thereof, in the region of the baflle 4. This height increase is brought about by dropping the bottom 6 of the crossover passage 5 below the level of the bottoms 31 of the left and right portions (halves) of tank 3. In operation, the tank 3 is partially filled with water 7, to a level sufliciently above the bottom of baffle 4 to prevent direct communication between the air spaces of the two halves of the tank 3; water communication between the tank halves can take place by way of crossover passage 5.

An air crossover duct 8, which may be tubular in crosssection (FIG. 5), is connected between the air spaces of the two halves of tank 3; duct 8 spans the partition or baffle 4. The duct 8 is of inverted U-shaped seen in elevation, and the two ends of the duct are sealed into the top wall of tank 3, at respective opposite sides of bafile 4. A removable orifice plate 9 (shown schematically in FIG. 1, but to be described more completely in connection with FIGS. 2-4) is sealingly mounted transversely in duct 8, in such a manner that all of the air flowing through duct 8 must necessarily pass through the central orifice 10 in this plate.

In operation, the tank 3 is partially filled with water 7, as previously stated. As the seaway 2 causes the ship 1 to roll, the water will tend to run through passage 5 from side to side of the tank, in its own natural period. The tank 3 is completely closed, that is, sealed otf entirely from the atmosphere. Because of the air-tight baffle plate 4, the air displaced by water running to one side of the tank must pass to the other side through the air crossover duct 8. By restricting the flow of air through duct 8 by means of the orifice 10 therein, a tuning of the frequency of water transfer in the tank may be accomplished, since the flow rate of the air being displaced in the closed tank governs the flow rate of the water which is displacing the air. Roll stabilization is effected when the natural frequency of water transfer in the tank 3 is equal to the rolling frequency of the ship, and when the oscillations of the water in the tank are 90 out of phase with the rolling movement of the ship. Under these conditions, there results a weight of water (and corresponding stabilizing moment) in a direction opposite to that in which the ship is tending to roll. This is illustrated in FIG. 1, wherein the ship 1 is level but tending to roll toward the right, as indicated by arrow A. The slope of the water surface 11 in the tank provides a weight of water on the left-hand side of the tank (and thus a stabilizing moment) which opposes the tendency of the ship to roll toward the right.

A ships natural rolling frequency varies with cargo loading. As previously stated, in order to effect optimum stabilization (thereby to eliminate the greater portion of the high amplitude rolling) the natural frequency of the tank should be equal to the natural rolling frequency of the ship. The stabilization system of this invention is readily tunable to suit each loading condition of the ship. This is achieved by making the orifice plate 9 removable (as will be detailed hereinafter), and by carrying a set of orifice plates, having dilferent-sized orifices, aboard the ship. Then, at the time of each change of loading, the proper sized orifice 10 may be provided in duct 8 to match the tanks natural frequency to the rolling frequency of the ship. In this connection, it will be appreciated that the flow rate of the air through duct 8 governs the flow rate of the water through crossover passage 5, and thus governs the tanks natural frequency; since the flow rate of the air through duct 8 depends upon the diameter of the orifice therein, changing the size of this orifice will change the tanks natural frequency.

FIGS. 2-4 illustrate the construction of a typical orifice plate arrangement with a removable orifice plate. FIG. 2 is a detailed view, on a larger scale, of the construction within the circled area B of FIG. 1. A short piece is removed from the center of duct 5 to leave a gap therein. A transversely-extending trapezoidal plate 12 having a central circular aperture 13 therein is welded to duct 8 at one end of this gap, the dimensions of plate 12 being somewhat in excess of the diameter of duct 8. Similarly, a tranversely-extending trapezoidal plate 14 having a central circular aperture 15 therein is Welded to duct 8 at the other end of the gap formed therein, the dimensions of plate 14 being somewhat in excess of the diameter of duct 8 (see FIG. 3). The diameters of apertures 13 and 15 are equal to each other, and are both just slightly less than the diameter of tubular duct 8. A side plate 16 (FIG. 3) is welded across one pair of aligned side edges of plates 12 and 14, and a side plate 17 is welded across the other pair of aligned side edges of plates 12 and 14, both of the plates 16 and 17 being located outside of duct 8. A bottom plate 18 is welded across the aligned bottom sides of plates 12 and 14, outside of duct 8. The plates 12, 14, 16, 17, and 18 thus provide a fixed open-topped box-like housing located in the central gap formed in duct 8.

An outwardly-extending horizontal flange 19 is welded to the upper end of plate 12, the opposite ends of this flange extending beyond the respective side plates 16 and 17 Similarly, an outwardly-extending horizontal flange 20 is welded to the upper end of plate 14, the opposite ends of this flange extending beyond the respective side plates 16 and 17. A set of studs 21 (illustrated as three in number in FIG. 3) is secured to flange 20, these studs extending upwardly from this flange. A similar set of studs 22 is secured to flange 19, these studs extending upwardly from this flange.

A trapezoidal orifice plate 9 having a central orifice 10 therein is welded to one face of a rectangular mounting plate 23 to form an orifice plate assembly 24 (FIG. 4). The plate 23 has such dimensions as to cover the open top of the fixed box-like housing previously referred to, plus the extensions formed at the top by flanges 19 and 20. The orifice plate assembly 24 is designed to be removably mounted within the said box-like housing, between plates 12 and 14 and with the orifice plate 9 parallel to plates 12 and 14. To mount the assembly 24 within the housing, plate 23 has six mounting holes 25 therein, each positioned to be aligned with a respective one of the studs 21 or 22 when assembly 24 is mounted within the housing. When assembly 24 has been positioned within the housing, wing nuts 26 are threaded onto the upper ends of the respective studs 21, 22 and are screwed down to bear against the upper face of plate 23, thereby to securely (yet removably) mount assembly 24 in the housing 12, 14, 16, 17, 18.

Gaskets are provided for sealing around the edges of the orifice in an air-tight manner. Thus, a gasket 27 having a rectangular outer configuration is provided for sealing at the upper end of assembly 24, between the bottom face of plate 23 and the upper face of flanges 19 and 20. A gasket 28 is utilized between the bottom edge of orifice plate 9 and the upper face of bottom plate 18. A gasket 29 is utilized between one side edge of orifice plate '9 and the adjacent inner face of side plate 17, and a gasket 30 is utilized between the other side edge of orifice plate 9 and the adjacent inner face of side plate 16. Thus, it may be seen that the orifice plate assembly 24 is removably mounted within duct 8, but in an air-tight manner, so that all of the air flowing through duct 8 must necessarily pass through the central orifice 10 in plate 9.

According to this invention, a set of orifice plate assemblies quite similar to assembly 24 described are carried aboard the ship, each assembly including an orifice plate quite similar to plate 9 but each plate having a different-sized orifice therein. At the time of each change of loading of the ship (which change causes a variation or change in the ships natural rolling frequency), the orifice plate assembly then mounted in the duct 8 is removed, and the proper-sized orifice plate assembly (i.e., the assembly having an Orifice of the proper size) is sealingly mounted in the box-like housing (previously described) provided in the air crossover duct 8-. From the previous description, it should be apparent how this removal of one orifice plate assembly and the insertion of another is effected. The term proper-sized orifice, of course, refers to one which will cause the tanks natural frequency to be closely matched to the ships natural rolling frequency (which latter, as previously stated, is variable with the cargo loading of the ship). It is not necessary that the tanks and ships natural frequencies be precisely equal, but they should be so closely matched that the difference will have negligible effect. The degree of matching, of course, will be a function of the number of orifice plate assemblies carried aboard the ship. It should be apparent that the stabilization system described may be simply and easily (and to a high degree of precision, since the diameter of each fixed orifice is accurately known or determinable) tuned to match each loading condition of the ship.

The orifice plate arrangement described is quite simple and inexpensive (as compared to a valve in the air crossover duct), is not subject to wear (as is a valve), and is very precise (as contrasted to a valve, in which the amount of opening is uncertain).

As previously mentioned, the bottom 6 of the water crossover passage is dropped below the level of the bottoms 31 of the two halves of tank 3. This reduces the total weight of water necessary in the tank 3. Since the water level may not be allowed to go below the bottom of the baffie 4 (if it did so, there would be a passageway for air here, and the action of the air crossover duct 8 would then be defeated), and since a sufficiently large passage must be maintained to allow free flow of water, prior systems have required a large margin of water depth to maintain the air seal. In the tank of the present invention, this water margin is minimized as a result of the dropped bottom 6. This feature of the invention may also be explained in another way. The portion of the water between the bottom of the tank and the level corresponding to the top of the water crossover duct is water which serves no really useful purpose. In the FIG. 1 design, the top of the water crossover duct is at the bottom edge of bafile 4. By dropping the water crossover passage at 6, the bottom edge of the bafiie 4 may be brought down close to the bottom 31 of the tank, thus minimizing the portion of the water which serves no really useful purpose, and yet a good-sized water crossover passage is provided.

Now refer to FIG. 6, which dislcoses a modified construction. In this modified construction, several air crossover ducts are used; three such ducts 8, 8, and 8" are illustrated in FIG. 6. Each of the crossover ducts 8, 8, and 8" is provided with an arrangement 32 (schematically illustrated in FIG. 6) similar to that previously described in connection with FIGS. 2-4, whereby an orifice plate assembly similar to assembly 24 may be removably inserted in each respective air crossover duct. Note that the arrangements 32 are similar in outer configuration to the showing in FIGS. 3 and 4. The construction of FIG. 6 enables combinations of various orifice sizes or blank plates to be effected; in this way, by combinations of a few standard orifice sizes or blank plates, many combinations of tank tuning may be achieved. By way of illustration, the orifice plate assembly mounted in duct 8 includes an orifice plate 9 having an orifice 10 of about the same size as that illustrated in FIG. 4. The orifice plate assembly mounted in duct 8' includes an orifice plate 9' having a larger orifice 10', while the orifice plate assembly mounted in duct 8" includes a plate 9" which is blank (i.e., it is imperforate, so has no orifice therein).

Refer now to FIGS. 7 and 8, which disclose another modification. Separate wing tanks 33 and 34 situated at the respective opposite sides of the ship 1 (the wings of a ship being considered to be the intersections of the hull with the upper deck) are coupled together at their lower ends or bottoms by a water crossover duct or pipe 35, and at their tops by air crossover duct 3. A removable orifice plate 9 is removably mounted in duct 8, as previously described in connection with FIGS. 14. The construction illustrated in FIG. 7 (wherein a removable orifice plate is again utilized) allows tuning of the natural frequency of the tank system to match the ships natural rolling frequency, for each loading condition of the ship, as described hereinabove.

The FIG. 7 construction enables the maximum stabilizing moment possible (for the weight of the water used) to be obtained, since in this construction the bulk of the water is concentrated at the sides of the ship, and thus at the maximum distance from the center of rotation thereof. Also, since the water crossover passage 35 is located at the bottoms of the tanks (and thus below one or more decks of the ship), the FIG. 7 embodiment allows fore and aft passage in the ship between the tanks 33 and 34.

In the FIG. 7 embodiment, there may be only a single air crossover duct, as previously described in connection with FIG. 1. Alternatively, as illustrated in plan in FIG. 8, there may be two or more air crossover ducts (such as 8 and 8') connected between the two wing tanks 33 and 34, each such air duct having therein a separate arrangement 32 for removably mounting an orifice plate therein, as previously described in connection with FIG. 6.

The invention claimed is:

1. A stabilization system for a marine vessel comprising a single elongate enclosure extending transversely across a marine vessel to be stabilized, the central region of the bottom wall of said enclosure being depressed below the level of the remainder of said bottom wall, a body of liquid in said enclosure having a liquid level such that in a repose horizontal condition an air space is provided above said liquid; an imperforate baffle plate centrally mounted in said enclosure and extending downwardly from the upper wall thereof for a limited distance into said body of liquid, thereby separating said air space only into two approximately equal portions, an air duct connected between said two air space portions, and an orifice plate removably mounted in said duct for throttling the passage of air therethrough.

2. System set forth in claim 1, wherein a plurality of air ducts are connected between said two air space portions, each such duct having a respective orifice plate removably mounted therein for throttling the passage of air through OTHER REFERENCES the corresponding duct.

References Cited UNITED STATES PATENTS 5 MILTON BUCHLER, Przmary Exammer.

2,585,290 2/1952 Walker 73-211 T. M. BLIX, Assistant Examiner. 3,269,344 8/1966 Bell 114125 Ser. No. 132,695 (A.P.C.), published May 11, 1943. 

1. A STABILIZATING SYSTEM FOR A MARINE VESSEL COMPRISING A SINGLE ELONGATE ENCLOSURE EXTENDING TRANSVERSELY ACROSS A MARINE VESSEL TO BE STABILIZED, THE CENTRAL REGION OF THE BOTTOM WALL OF SAID ENCLOSURE BEING DEPRESSED BELOW THE LEVEL OF THE REMAINDER OF SAID BOTTOM WALL, A BODY OF LIQUID IN SAID ENCLOSURE HAVING A LIQUID LEVEL SUCH THAT IN A REPOSE HORIZONTAL CONDITION AN AIR SPACE IS PROVIDED ABOVE SAID LIQUID; AN IMPERFORATE BAFFLE PLATE CENTRALLY MOUNTED IN SAID ENCLOSURED SAID EXTENDING DOWNWARDLY FROM THE UPPER WALL THEREOF FOR A LIMITED DISTANCE INTO SAID BODY OF LIQUID, THEREBY SEPARATING SAID AIR SPACE 